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Fig. 5. Proposed hypothetical model of active ammonia excretion across gills of the
shore crab Carcinus maenas. According to this model,
NH4+ is pumped across the basolateral membrane by
Na+/K+-ATPase (1) or traverses the membrane via
Cs+-sensitive channels (2). Dissociation of cytosolic
NH4+ to H+ and NH3 is accompanied
by diffusion of NH3 into vesicles acidified by V-type
H+-ATPase (3). The ammonia-loaded vesicles (4) then are moved
via microtubules (5) to the apical membrane where vesicles fuse with
the external membrane, releasing NH4+ into the
subcuticular space. The NH4+ then is believed to diffuse
across the cuticle, via amiloride-sensitive structures (6). Role and
localization of the putative ammonia transporter (RhCM) identified in
Carcinus maenas gill epithelium (GenBank Accession number: AF364404)
are presently uncharacterized (7). Paracellular ammonia diffusion (8) and
non-ionic transcellular diffusion of NH3 (9) is considered to be
low under physiologically meaningful transepithelial ammonia gradients
(Weihrauch et al., 1998).
(Modified after Weihrauch et al.,
2002.)
